1. Field of the Invention
This invention relates to elements for protecting electrical circuits and, more particularly, to a fuse element having a fusing characteristic which is controlled to give maximum protection from destructive currents without unnecessarily responding to safe overload current.
2. Prior Art
Fuses, or the fusing elements in the fuses, commonly found in electrical circuits have varying fusing times versus current levels, as can be seen in the curves of FIG. 1. In FIG. 1, Curve A shows the fusing time versus current for a fusing element of medium width. Curve B shows the fusing time versus current for a wide fuse element and Curve C shows a fusing time versus current for a fusing element having a constricted section. As can be seen in FIG. 1, a fuse opening time for the narrow width uniform strip-like element b-1 falls off dramatically in the high current region I. In the low-current region II the fuse opening time for the broad-width, uniform, strip-like fusing element b-2 rises marketedly. The fusing element b-3, having a constricted portion of length L, has a fusing time-current characteristic which overlaps the characteristics of elements b-1 and b-2. A short fusing time is desirable for destructive surges of current whereas a longer fusing time is required in connection with tolerable over-loads. It is not desirable to disconnect the load from the power supply for minor over-load conditions since it interrupts the operation of the equipment and may be destructive. Thus we have the inconsistent and contradictory requirements of avoidance of unnecessary fusing in the simple over-load state while at the same time requiring rapid fuse opening upon the flow of a destructively large current surge.
Furthermore, since the restricted portion such as that shown as b-3 in FIG. 1 has to withstand the potential appearing across the fuse terminals after fuse opening has occurred there is a minimum length L for the restricted region b-3.
In the past the problem of heat dissipation to prevent unnecessary fuse opening was accomplished by inserting the broad portions X and Y of the fusing element 10 in FIG. 1 into a heat dissipating member made of heat resistant ceramic and bonding the fusing element and the ceramic holder together by means of adhesives. However, such a structure has the drawback that it is large and, because the heat-dissipation member is made of ceramic the manufacturing cost is high. Because of the size of such a structure it is not freely usable in environments such as vehicles. There large acceleration forces may be experienced as a result of mechanical vibrations with the result that there is a separation between the fusing element and its ceramic support, thereby mechanically destroying the fuse.
Other efforts have been made to increase the heat dissipation in the fusing element by using a low melting point alloy which is applied to the broad-width portion of element 10, or a thick layer of low melting point alloy is formed between two sheets of metal. With this approach it is intended that the fusing time will be increased by increasing the surface or the volume of the broad-width portion of element 10. The increase in fusing time is particularly desirable at the boundary between the low current region II and the medium current region III to prevent the fusing time at a low current region from being increased. Looking at that another way, the fusing time is long in the low current region I. An eutectic alloy is formed between the low melting point metal and the other material which lies on both sides of that low melting point metal, thereby preventing an increase in the fuse opening time which might result were it not for a lower melting point of the eutectic alloy. At the boundary between the low current region I and the medium current region III, since the fuse opening time is relatively short, the eutectic alloy does not effectively increase the fuse opening time. However, the combination does avoid unnecessary fuse opening during over-load operation. With this combination, however, the material costs are high and the fabrication costs are high. Further, since weight has been added to the fusing element, that element may possibly be destroyed mechanically by any large force applied to the junction between the fusing element and the fuse terminals.
Therefore, it is an object of this invention to overcome the various disadvantages of the prior art devices.
It is a further object of this invention to provide a fuse with improved fusing characteristics and improved mechanical properties.